Generally, a magnetic recording medium comprising a non-magnetic support having thereon a magnetic layer containing ferromagnetic particles dispersed in a binder is used as a magnetic recording medium for audio recording, video recording and for computers.
Carbon black is conventionally contained in the magnetic layer of such a magnetic recording medium for preventing the static electrification of the magnetic layer. An average primary particle diameter of carbon black is generally from 10 to 150 m.mu. (milli micron). It is generally considered that when carbon black having a small particle size is used, the surface of the thus obtained magnetic layer becomes smooth, but running durability becomes poor. On the other hand, when carbon black having a large particle size is used, running durability becomes excellent but surface smoothness becomes poor, which results in undesirably deteriorating electromagnetic properties and particularly S/N in the case of a video tape. For solving the above problem, it has been proposed in Japanese Patent Publication Nos. 9041/79, 20203/78 and Japanese Patent Application (OPI) No. 218039/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application"), that carbon black having different average primary particle sizes be added in a magnetic layer when the magnetic layer is a single layer, but a magnetic recording medium having both satisfactory running durability and electromagnetic properties can not be obtained.
On the other hand, when a magnetic layer is a multiple layer having at least two layers, a method for preparing a magnetic recording medium having excellent running durability as well as excellent electromagnetic properties is disclosed in Japanese Patent Application (OPI) No. 200425/83. That is, the magnetic layer is composed of two magnetic layers, and only the second magnetic layer (upper layer) contains carbon black to improve running durability. The first magnetic layer (under layer) does not contain carbon black so as to increase packing density of magnetic particles in magnetic layer, whereby electromagnetic properties are improved. However, the above described magnetic tape has such small carbon black particles (i.e., average primary particle diameter is 30 m.mu. ) contained only in the upper layer and satisfactory running durability cannot yet be obtained.
Accordingly, it cannot be said that the above method can produce a magnetic recording medium sufficiently excellent in running durability as well as in electromagnetic properties.